Expensive helmet won't protect you any better. As long as it has the label inside specifying that it has been certified to national safety standards, it's as safe as the next one. It's about weight and venting.

When I got my bike, I got a $60 helmet, which was fine. After a few years of riding I found it really hot in the summer, saved up and got Giro Atmos (on sale at the end of the season is was $200, crazy, I know but not on sale it was $350). I love it, it's very light and vents well. To me it was worth the money, because I got exactly what I wanted, and I've been using that helmet for 4 years now. The only thing is, with more vents, there's more places for bugs to get caught, when you're riding. I still keep my old helmet for cold day rides, as it fits the little cap underneath very well.